Recently, progressive-power lenses having various types of refracting surfaces are being sold in the market.
The progressive-power lenses can be classified according to type, configuration and the like of the refracting surfaces thereof. For example, there are a convex surface progressive-power lens having a progressive-power surface (a curved surface having progressive-power) arranged on the convex surface side, a concave surface progressive-power lens having a progressive-power surface arranged on the concave surface side, a both-surface progressive-power lens having a progressive-power surface arranged on both surfaces, and an integrated-double surface progressive lens where the progressive-power is divided into a horizontal progressive element and a vertical progressive element, wherein the horizontal progressive element is owned by the convex surface side and the vertical progressive element is owned by the concave surface side.
Further, vision area of the progressive-power lens can be broadly divided, based on the distance to a visual target, into three types, which are distance vision type, intermediate vision type and near vision type. However, the vision area of the progressive-power lens may also be divided based on what vision area is emphasized. For example, there are a distance vision emphasized type progressive-power lens, an intermediate vision emphasized type progressive-power lens, a near vision emphasized type progressive-power lens, a distance and intermediate visions emphasized type progressive-power lens, an intermediate and near visions emphasized type progressive-power lens, a distance and near visions emphasized type progressive-power lens, and a distance, intermediate and near visions balanced type progressive-power lens. Further, there is another type of progressive-power lens called “individual progressive-power lens”, which is made corresponding not only to general information, such as prescribed power and the like, but also to information other than the general information, such as the distance between the eyeballs of the wearer and the lenses, the tilt angle of the frame, and the line-of-sight of the wearer when the wearer views an object (the habit of the wearer). In such a manner, various designs of the progressive-power lenses are being developed, and diversity is significantly increasing especially in recent years.
In order to reduce the processing time and processing cost, a method for processing the convex surface progressive-power lens, which is the most popular progressive-power lens among the various popular progressive-power lenses mentioned above, is widely used in which the progressive-power surface on the convex surface side of the lens is previously processed, so that only the concave surface side needs to be processed after receiving an order. In such a case, the lens with the convex surface previously processed is particularly called a “semi-finished lens” (abbreviated as “semi” hereinafter).
The various types of progressive-power surfaces mentioned above are designed based on various design concepts by a computer having a lens design program incorporated therein to concretely determine a three-dimensional shape. Generally, the three-dimensional shape is achieved by performing processing using a machine tool called a “numerical control (NC) processing machine”. The progressive-power surface can be formed by directly processing the lens material, however, the progressive-power surface is generally formed on a mold or a matrix for molding the mold, so that the semis with stable progressive-power surfaces can be mass-produced by molding (see Patent Document 1).
The process of processing the progressive-power surface by using the NC processing machine is generally called “free-form processing” (see, for example, Patent Document 3, Patent Document 4 and the like).
To process the progressive-power lens after receiving an order, a semi having a suitable base curve (i.e., a curve of a distance portion of the progressive-power surface, also referred to as a “BC”) is selected from the inventory of the various mass-produced semis using a previously prepared classification table according to the prescribed power of the wearer, and then a curved surface on the concave surface side is designed and processed using the selected semi so as to meet the prescribed power of the wearer. In such a case, the curved surface on the concave surface side is a relatively simple curved surface such as a spherical surface, a cylindrical surface and the like, and therefore the processing of the curved surface on the concave surface side is far easier than that on the convex surface side. In other words, the convex surface is a progressive-power surface containing addition power elements, while the concave surface is configured as a prescription surface where the other prescription values than the addition power of the entire lens is added, the other prescription values including the cylindrical power, the cylinder axis and the like.
Note that the term “cylindrical power” used in the whole description means a far-sight cylindrical power.
FIG. 11 is a table showing an example of a manufacture range and BC classification of a typical progressive-power lens having a progressive-power surface formed on the convex surface side.
In other words, the manufacture range of FIG. 11 is as the follows:
SPH (spherical power): +8.00˜−10.00, and
CYL (cylindrical power): 0.00˜−4.00
(SPH+CYL≧−10.00)
The unit of the respective values is diopter (D), and the manufacturing pitch is 0.25 diopter for all items
Thus, in FIG. 11, the number of combinations of the SPH and CYL is 1,105, which is equal to the number of the grids of the manufacture range of FIG. 11. Incidentally, “CYL=0.00” means the lens is a spherical lens.
Further, as an example of a typical manufacture range and BC classification, in the case where the number of BC is 5, the number of addition power is 12 (wherein the addition power changes from 0.75 to 3.50 at a pitch of 0.25 diopter), and the lens are respectively designed for both the right eye and left eye, the total number of the kinds of the lenses calculated as follows have to be prepared as semi-finished products (referred to as “semi-finished lenses” or “semis” hereinafter):5(BC classifications)×12(addition power classifications)×2(left eye/right eye classifications)=120 kinds
Next, “curve classification” of the concave surface side of a typical progressive-power lens will be described below.
As shown in FIGS. 12 and 13, the “curve classification” corresponds to the BC classification of the convex surface side shown in FIG. 11, and represents a curve of the curved surface on the concave surface side. Further, FIG. 12 corresponds to a “curve in base direction” (i.e., a shallower curve (D2) on the concave surface side), and FIG. 13 corresponds to a “curve in cross direction” (i.e., a deeper curve (D3) on the convex surface side). Incidentally, the “curve in base direction” is generally called “base curve” as opposed to “cross curve”, however, in the present description, to avoid confusion with the term “BC” on the convex surface side, the “curve in base direction” is called as it is.
Herein, the concave surface within a spherical power range is a spherical surface where “D2=D3”, the concave surface within a cylindrical power range is a spherical surface where “D2<D3”, and cylindrical power CYL is indicated by “D2−D3”. In other words, the concave surface within a cylindrical power range has two kinds of different sectional curves D2 and D3, and the direction of the sectional curves D2 is generally called an axial direction of cylindrical power (AXIS). The case of D2<D3 is taken as an example so that CYL<0.00, however, in the case where CYL>0.00, the relation of D2 and D3 will be D2<D3.
Here, the relation between SPH (spherical power), CYL (cylindrical power), BC described in FIG. 11, D2 described in FIG. 12, and D3 described in FIG. 13 can be expressed by the following Equations (1) and (2) as approximate values where the effect of the thickness of the lens is ignored:SPH=BC−D2  (1)SPH+CYL=BC−D3  (2)
Further, the following Equation (3) can be obtained by subtracting Equations (1) from Equations (2):CYL=D2−D3  (3)
It cab be obviously known from Equations (1) and (2) that the base direction (D2) is a direction of a lens cross-section having “SPH (spherical power)”, and the cross direction (D3) is a direction of a lens cross-section having “SPH (spherical power)+CYL (cylindrical power)”.
Generally, the axial direction (AXIS) of the cylindrical power is used to express the base direction in degrees measured anti-clockwise from the horizontal right direction, wherein the horizontal right direction is 0°, and the angle varies in a range of 0°˜180° in increments of 1°.
Incidentally, the direction of 0° is often expressed as 180°, which represents the same direction. Further, the direction of 180°˜360° is generally expressed by a value obtained by subtracting 180° therefrom.
Next, various kinds of the concave surfaces of the typical progressive-power lenses are shown in FIG. 14. FIG. 14 is a table showing a list of combinations of FIG. 12 and FIG. 13, wherein the vertical axis represents the value of D2, and the horizontal axis represents the value of D3 in the case where D3 is equal to CYL. The total number of the kinds of the concave surfaces shown in FIG. 14 is 561 kinds. Generally, in the case of the typical progressive-power lenses, tools (processing plates) respectively corresponding to concave surfaces shown in FIG. 14 are previously prepared to cope with various orders.
Specifically, to process a typical progressive-power lens, a semi having a suitable BC and addition power (ADD, for right eye or left eye) is selected from the inventory of various semis using a previously prepared classification table according to the prescribed power (SPH power and CYL power) of the order, and a curved surface having D2 and D3 on the concave surface side is designed and processed using the selected semi to meet the prescribed power of the wearer. A concrete method for processing the concave surface side is generally used in which a rough grinding process is performed using a machine called a curve generator (CG), and a sanding process and/or polishing process is performed using a convex-shaped tool (a processing plate) having a curve conforming the concave surface of the lens (see, for example, Patent Document 3 and Patent Document 4).    [Patent Document 1] International Patent Publication WO 98/16862    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2003-84244    [Patent Document 3] International Patent Publication WO 2005/084885    [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2006-312233